As a typical scanning probe microscope (SPM), a scanning tunneling microscope (STM) and an atomic force microscope (AFM) have been known. Among them, AFM has a cantilever having a probe at a free end, a displacement sensor for detecting displacement of the cantilever, and a sample stage scanner. In AFM, the cantilever is oscillated at a frequency near resonance frequency by oscillating a piezoelectric element, and a probe of the oscillating cantilever is contacted to a sample. Oscillation amplitude of the cantilever is decreased by contact with the sample. Based on output of the displacement sensor, the cantilever and the sample are relatively scanned with a decreased amplitude amount being kept.
To keep the decreased amplitude amount constant, an amplitude target value (set-point) is set. The amplitude target value is set slightly small compared with free oscillation amplitude. For example, the amplitude target value is set 0.9 times as large as the free oscillation amplitude. An amplitude value is detected by the displacement sensor during scanning a sample stage in X and Y directions, and the sample stage is subjected to feedback control in a vertical direction (Z direction) such that the amplitude value corresponds to the amplitude target value.
When the amplitude value is smaller than the amplitude target value, AFM determines that the probe is excessively close to the sample, and moves the sample stage to be away from the cantilever. On the other hand, when the amplitude value is larger than the amplitude target value, AFM determines that the probe is excessively away from the sample, and moves the sample stage close to the cantilever. By such feedback scan, a distance between the sample and the probe is kept constant, and information such as surface morphology of the sample is obtained by a feedback signal or the like.
In such feedback scan, excitation efficiency of the cantilever is supposed to be constant. The excitation efficiency is conceptually equal to magnitude of actual oscillation intensity (amplitude “A”) of the cantilever against excitation intensity (amplitude “A0”) of an excitation source, and can be expressed by an amplitude ratio (kA/A0, k is constant). The excitation source is, for example, the piezoelectric element excitation source. Actual amplitude of the cantilever is influenced by the cantilever and other relevant elements. For example, in use of the cantilever in a liquid, the amplitude of the cantilever is influenced by the surrounding liquid. Based on such various factors, the amplitude of the cantilever is determined, and excitation efficiency is also determined.
However, it is known that the excitation efficiency varies during scan in actual AFM. For example, when the excitation efficiency decreases, amplitude of the cantilever is decreased. In this case, AFM determines that the probe is excessively close to the sample, and moves the sample stage to be away from the cantilever. As the excitation efficiency decreases, the probe of the cantilever is more moved to be away from a surface of the sample, eventually the probe is perfectly separated from the surface of the sample, and as a result, the AFM can not perform imaging.
Such drift of excitation efficiency is the largest problem among various types of drift. Imaging for long time (several minutes) is difficult because of the drift. The problem exists in both a fast-type atomic force microscope and a typical atomic force microscope.
Conventionally, when the excitation efficiency is tried to be detected, the oscillation amplitude needs to be measured while the probe is perfectly separated from a surface of the sample to obtain the free oscillation amplitude. The measurement can not be performed during actual imaging scan. Therefore, even if the conventional detection method is applied, the drift of the excitation efficiency can not be corrected during scan.
When the drift of the excitation efficiency occurs, a relationship between amplitude and a target value of the amplitude varies, and as a result, imaging is adversely influenced as described above. To cope with such a problem, Schiener et al. perform PI control to the amplitude target value based on a fact that an amplitude signal of second harmonic resonance (a component of frequency twice as high as the primary resonance frequency) of the cantilever is sensitive to intensity of contact between the probe and the sample (Schiener et al., “Stabilized atomic force microscopy imaging in liquids using second harmonic of cantilever motion for set-point control”, Review Of Scientific Instrument, American Institute of Physics, August 2004, Volume 75, Number 8, pp. 2564-2568). However, the method is disadvantageous in that as a result of changing a target value, force exerted between the probe and the sample is changed during scan.
Among types of AFM, non-contact AFM is known in addition to contact AFM. The non-contact AFM is used with the probe being close to the sample. The problem of excitation efficiency is not a problem only in the contact AFM. The same problem may occur in the non-contact AFM.